The present invention relates to the field of wireless communications systems, and in particular, to a method and system for uplink and/or downlink prediction in wireless systems employing adaptive antenna arrays.
Although antennas have sometimes been one of the neglected components of wireless systems, relatively recent developments in the field of (adaptive) antenna arrays and xe2x80x9csmart antennasxe2x80x9d have realized significant improvements in various aspects of wireless communication, such as frequency spectrum use, signal-to-noise ratio (SNR), interference reduction, directionality, spatial diversity, power efficiency, and security. Antenna arrays may be employed in a number of different wireless applications, including, but not limited to, radio communication systems, cellular systems, television broadcasting, point to point systems, paging systems, medical applications, etc).
Antenna arrays typically include a number of antennas that are spatially separated and coupled to one or more processors. Adaptive antenna arrays, or simply, adaptive arrays, periodically analyze the signals received from each of the antennas in an array to distinguish between desired signals (e.g., from a desired source, such as cellular telephone or other communication device) and undesired signals (e.g., interference sources), multipath, etc. The process of combining the signals from a number of antenna elements to enhance the gain toward a desired source is sometimes referred to as beamforming.
The use of a number of spatially separated antennas allows adaptive array systems to transmit and receive signals in a spatially (and/or temporally) selective manner. As such, adaptive arrays perform beamforming in such a way as to enhance their transmit (downlink) and receive (uplink) energy xe2x80x9ctowardxe2x80x9d a xe2x80x9cdesiredxe2x80x9d source, while diminishing transmit and receive energy toward interfering sources. Because adaptive array systems may distinguish between spatially distinct sources (e.g., two cellular user units separated in space), such systems are sometimes referred to as xe2x80x9cspatial processingxe2x80x9d or xe2x80x9cspatial division multiple access (SDMA)xe2x80x9d systems. Such systems generally provide improved performance relative to single antenna element systems.
FIG. 1 is a diagram depicting a simplified radiation pattern of an antenna array system. As shown, in accordance with known techniques, an antenna array 10 transmits and/or receives signals with a desired source 12, representing a remote user terminal, (e.g., a mobile or stationary communicator, a modem, or other wireless communication device) by generating an enhanced gain region 18, representing a relatively enhanced radiation gain pattern for the antenna array 10. The enhanced gain region 18 is directed toward, or more generally, associated with, the desired source 12 for transferring signals between the antenna array 10 and the desired source 12.
On the other hand, the antenna array 10 generates an interference mitigated region 16, sometimes referred to as a xe2x80x9cnullxe2x80x9d region, directed toward an interfering source 14 (which may be another remote terminal or environmental interference source, such as a moving vehicle). Contrary to the enhanced gain region 18, the interference mitigated or null region 16 represents a region of relatively minimal radiation gain.
It should be appreciated that the interference mitigated region 16 may often include some level of gain, though typically less than the enhanced gain region 18. Ideally, an antenna array would generate a null to direct zero gain toward an interfering source. Furthermore, it should be appreciated that FIG. 1 shows a simplified depiction of a radiation pattern of only one type of antenna array system. Therefore, the discussion herein is not limited to any one type of antenna array or spatial processing technique or radiation pattern or beamforming technique, or a particular wireless system or application.
Typically, the antenna array 10, during uplink communications (i.e., communications transmitted by the desired source to the antenna array), applies an amplitude and phase adjustment to each uplink signal received at each array element to enhance (uplink) gain toward the desired source(s)xe2x80x94in this case, the desired source 12xe2x80x94while minimizing undesired signals and noisexe2x80x94e.g., the interfering signals associated with the interfering source 14. By taking into consideration the amplitude and phase adjustments for all of the array elements, the antenna array 10 may compute an uplink xe2x80x9cweight,xe2x80x9d which may be a vector or matrix, the latter of which may have spatial and/or temporal components. In turn, the uplink weight determines the energy pattern for the antenna array 10 during uplink.
On the other hand, during downlink transmission (i.e., transmission from the antenna array to the desired source), the antenna array 10 may use the previous uplink weight (along with adjustments for calibration) for downlink beamforming. Ideally, a downlink weight allows an antenna array to enhance the transmission gain in the direction of the desired user, while minimizing the transmission gain in the direction of interferers, and an uplink weight allows an antenna array to enhance the reception gain in the direction of the desired user, while minimizing the reception gain in the direction of interferers.
Subsequent to uplink beamforming, the antenna array 10 typically will transmit to the desired source 12 (and mitigate interfering sources) in accordance with essentially the same weight computed for previous uplink signals. In other words, the gain pattern for the uplink signal(s) is utilized for, and thus, is mirrored by the next downlink gain pattern. Expressed in terms of the uplink weight used to derive the uplink beamform, the uplink weight at time k, represented by WUkis mirrored by the downlink weight at time k+t, such that WUk=WUk+t. In some applications, the uplink and downlink weights may not be identical, for example, due to calibration, as described, for example, in U.S. Pat. No. 6,037,898, entitled, xe2x80x9cMethod and Apparatus for Calibrating Radio Frequency Base Stations Using Antenna Arrays,xe2x80x9d issued Mar. 14, 2000 and assigned to the assignee of the present invention.
Although the method and system shown in FIG. 1 may provide sufficient performance in applications where the desired source 12 and the interfering source 14 are stationary or there are relatively few other interfering sources (e.g., moving cars, other communicating devices, etc.) or the number of total user terminals is relatively small, undesired results may occur in mobile environments. For example, in a cellular environment, where the antenna array 10 may be part of a basestation, the desired source 12 may be a mobile unit (e.g., a cellular phone) that is in motion, as may be one or more other interfering sources, using a downlink weight that merely mirrors the previous uplink weight may result in interference or other undesirable performance losses.
At least one reason for such performance loss is the time interval between uplink and downlink transmissions in some systems, for example, time division duplex (TDD) systems. Because signal sources, both desired and interfering, may be in motion and thus change their spatial location in such time interval, using a downlink that essentially mirrors a previous uplink may provide unsatisfactory performance. For instance, in the diagram shown in FIG. 1, if the desired and interfering sources are in close proximity and moving toward each other, the desired source may move closer to the interference mitigated region 16, while the interfering source may move closer to the enhanced gain region 18. As such, not only may the enhanced or maximized signal gain not be directed accurately toward a desired source, but also interference mitigated regions may not be directed to interfering sources, due to the motion.
Thus, what is desired is a method and system that overcomes the above-described limitations associated with uplink and downlink beamforming, especially in mobile environments.
A method and apparatus for predicting information for uplink and downlink beamforming in a wireless communication system employing an antenna array is provided. According to one aspect of the invention, a method and apparatus to predict uplink and/or downlink weights as a function of previous weights is provided. A sample of uplink weights and/or downlink weights are used to predict a future uplink or downlink weight.